Method of producing cellulosic pulp



' perior product.

Unite States Patent'O METHOD OF PRODUCING CELLULOSIC PULP Ernest R.Hatheway, 4625 Ravenwood Ave., Sacramento, Calif.

Filed Oct. 19, 1956, Ser. No. 616,969

4 Claims. (Cl. 16289) The invention relates to improvements in the sodaand the sulfate processes of making paper and other cellulosic pulps,and more particularly the production of alpha cellulose pulps in highyields and adapted for use in the chemical cellulose industry.

Historically, the soda process is one of the oldest methods of producingpaper pulp. In the latter years, however, the soda process has givenground to the sulfite, or acid process; and, more recently, both thesoda and sulfite processes have yielded, tonnagewise, to the sulfateprocess.

In all of the processes, the raw material, such as wood is appropriatelyprepared, as by reducing it to chips, the chips thereupon being cookedin a liquor at an elevated temperature and pressure, cooking beingeffected in a vessel termed a digester. Cooking is followed by washingand a series of pulp treating steps, and by bleaching.

At the conclusion of the bleaching step, the resultant pulp differsrather substantially depending on which of the processes is used. Thepulp produced from both the soda and sulfite processes can generally becharacterized as high white paper stock but only the stock resultingfrom use of the sulfite method can be further purified to yield alphacellulose in commercial quantities. The pulp obtained from the sulfateprocess cannot be classified as high white paper stock; instead, thesulfate produced pulp is used in the making of heavy, strong paper suchas that used in bags, cartons, etc.

Despite the numerous advantages of the soda and sulfate methods,collectively termed the .alkaline processes, it has heretofore not beenpossible, so far as is known, to produce a high white paper stock aswell as a high yield of alpha cellulose, anincreasingly important andvaluable raw material for the viscose and related industries, from boththe soda and the sulfate processes.

Furthermore, it has heretofore been necessary, in the production of goodquality paper stock and high grade alpha cellulose, to be especiallyselective in the wood used. Downgraded material has thus undergone asubstantial reduction in value with consequent. economic loss to theproducer.

It is therefore an object of the invention to provide a method ofproducing celluloses which retains the advantages of the well-known sodaand sulfate processes.

It is another object of the invention toprovide a method of producing ahigh yield of good quality celluloses.

It is still another object of the invention to provide a method ofbleaching cellulosic material to a high degree of brightness andpermanence without significant loss in strength.

It is yet another object of the invention to provide a method ofproducing celluloses which is applicable not only to wood materials ofall grades but also to wood wastes and to annual crop products, such asstraw, rice hulls, bagasse, corncobs, etc.

It is another object of the invention to provide a more economicalprocess and yet one which results in a en- 2,962,413 Patented Nov. 29,1960 ICC It is still another object of the invention to provide a methodof producing a high yield of substantially pure alpha cellulose pulp andhigh white paper stock.

It is a further object of the invention to provide a generally improvedmethod of producing celluloses.

Other objects, together with the foregoing, are attained in the methoddescribed in the following description and shown in the accompanyingdrawing in which a diagrammatic representation of the method appears.

In general, the drawing shows a conventional alkaline process flowchart; and, in the following description, the terminology employedfollows the terminology long-used and wholly familiar to those skilledin the art. Consequently, unless the description calls for procedures,equipment, concentrations, reagents, temperatures, pressures, etc.,differing from those presently employed in the soda and sulfate process,the traditional methods are presumed to be followed.

The process of my invention results not only in producing high whitepaper stock at a stageconsiderably in advance of that required in thepresent day soda process and sulfate processes, but also in producing apulp having an alpha cellulose content in excess of 96% and an ashcontent of less than 0.1%, a product far superior to that realized inthe conventional alkaline processes. Furthermore, by appropriatemodification of some of the variables, high white paper stock and alphacellulose yield is obtainable from annual agricultural wasteplantmaterials such as straw, corncobs, and bagasse; and, by furthermodification, even rice hulls can be reduced to valuable cellulosicproducts, a result heretofore not achieved by any well recognizedprocess, so far as is known, rice hulls being extremely difficult toprocess owing partly to their high (about 20% by weight) content ofsilica.

Exhaustive experiments have been undertaken to verify and establish thelimits herein. California redwood (Sequoia sempervirens) was chosen asthe subject material for the wood pulp, and rice hulls as the especialsubject for agricultural plant wastes. Each is considered particularlydiflicult to treat, and success in their use clearly indicates, it isfelt, a highly satisfactory product in the event other and more commonlyused raw materials are employed.

While each of the following described departures from the conventionalalkaline processes is believed to be of importance in achieving thefinal valuable result, it is felt that the step employed in the initialstage, wherein the material is digested or cooked in a liquor ofpredetermined composition, is perhaps the most important.

Whereas the cooking liquor for the soda process consists of a solutionof caustic soda containing a small amount of sodium carbonate(Sutermeister, Chemistry of Pulp and Paper Making, Third Edition, 1941,John Wiley & Sons, New York, p. 139), the cooking liquor of the processof the invention also includes a predetermined amount of sodium,calcium, or similar hypochlorite in relation to a predetermined amountof liquor alkali as well as a predetermined liquid to solid ratio, thatis to say, the ratio by weight of the cooking liquid to the dry weightof solid pulp material stock. In comparable fashion, the process of theinvention also com prehends the addition of a suitable hypochlorite tothe cooking or White liquor used in the sulfate process and which has asits principal active constituents sodium hydroxide and sulfide(Sutermeister, p. 107).

In the following examples, conventional sulfate liquors were used exceptas specific quantities of certain of the ingredients and additives arestated.

Example 1.-Redwood chips Cooking liquor was prepared, the liquor inaddition to other conventional sulfate liquor ingredients comprising 68grams per liter of sodium hydroxide, 22 grams per liter sodium sulfide,1 gram per liter sodium sulfate, and sodium hypochlorite in an amountequal to 1% of the Weight of the wood chips, with a liquid to solidratio of 5 to 1. Liquor and redwood chips were placed in a closed vesseland the temperature raised to 155 C. as rapidly as possible,approximately one hour. Temperature and vessel pressure were thereaftermaintained for approximately one hour.

Digester was thereupon blown and the product subjected to the customarywashing and screening operations.

Pulp was then subjected to chlorine bleaching, more than normalrequirements of chlorine being used to solubalize the residual ligninand remove color. Chlorine used was equal to 5% of dry weight of wood.Chlorinated pulp was thereupon subjected to a hot sodium hydroxideextraction and then washed.

Sodium hypochlorite equivalent to 1% of the dry wood weight was thenplaced in solution and the pulp introduced therein to efiect, afterwashing, a pulp of high White brightness.

At this stage the pulp constituted a high white paper stock and could beused as such. Further purification, now to be described, resulted in ahigh alpha cellulose pulp yield.

Purification consisted of slurry steeping (cold) in a sodium hydroxidesolution of 10% NaOH for 30 minutes. The solution was thereuponrecovered from the pulp by pressing, the pulp itself being subsequentlyshredded and then washed.

Final brightening was thereupon elfected by a warm wash slightlyacidified with E01 (S being equally satisfactory) and a subsequentfilter wash. The resultant product was a high white pulp having an alphacellulose content of 96% to 98% with an ash content less than 0.1%, thedesirable and valuable properties of the cellulose having beenprotected, or achieved, by the relative mildness of its digestion stagein contrast to the customary alkaline process type of cooking.

Depending on the wood, variant amounts of reagents, times andtemperatures are to be used. Thus, depending on the wood, the cookingliquor might contain 64 to 80 grams per liter of caustic, 1% to 2% ofwood weight of hypochlorite, and a liquid-solid ratio of 5 to 1 to 6to 1. Temperature of cooking can range from 155 C. to 165 C. dependingon the wood, with time in coming up to temperature varying from one hourto one hour and a quarter, and time of holding at temperature and vesselpressure ranging from one to one and one-half hours. In the chlorinationbleaching step, the amount of chlorine used may vary from 5% to 8% ofthe wood weight, again depending on the type of wood treated. The sodiumhydroxide, furthermore, used in the purification process can vary from10% to 12.5%, with the slurry steeping period ranging from 30 minutes toone hour.

The use of hypochlorite in the cooking liquor represents a radicaldeparture from classical alkaline process practice, the hypochloritepermitting the use of the less expensive alkaline processes in theproduction at an early stage of high white paper stock and the efiicientproduction of high grade alpha cellulose pulp, a result heretoforeconsidered unattainable economically in commercial yields.

The first reaction of hypochlorite in a highly alkaline cooking liquoris chlorination. This moderate chlorination at an early stage in thetreatment results in a more easily bleachable pulp. The amount ofhypochlorite used, as well as the time and temperature of the cook, mustbe controlled so that the maximum amount of available chlorine in thehypochlorite is directed to chlorination and as little as possible tooxidation. This situation is critical since hypochlorite under certainconditions has a high oxidation potential. Increasing the temperatureabove the maximums herein described results in marked degradation of thecellulose. The use of hypochlorite in the cooking stage, in addition toits other valuable contributions, permits of some degree of viscositycontrol, a factor of especial importance to the viscose and chemicalcellulose industry.

While the process can be applied to all woods and wood wastes, includingredwood and redwood bark, it has been found that, owing to the unusualproperties of redwood bark, digestion is preferably carried on withoutvessel pressure and at a temperature of C. to C.

In somewhat comparable fashion, in the case of all annual plant wastematerials, with the exception of rice hulls, digestion is preferablycarried on at atmospheric pressure, within a temperature range of 95 C.to 100 C. and at a liquid solid ratio of 10 to 1.

Example 2.Rice hulls Digestion was carried out using a cooking liquorcomprising, among other conventional sulfate liquor ingredients, 80grams per liter of NaOH, sodium hypochlorite equal to 2% of hull weight,with a liquid-solid ratio of 10 to 1. Time of coming up to C.temperature in closed vessel was one hour, with pressure beingmaintained for one hour and a half.

The charge was then blown and the pulp washed and screened, with rejectsgoing to a suitable refiner for later return.

Chlorine was thereupon introduced into the pulp under agitation for 30minutes at a rate of 0.25% of the weight of the hulls per minute.Chlorine flow was then shut off and agitation of the pulp continued for20 to 30 minutes to consume all free chlorine in the liquid medium.

After washing, the pulp was subjected to a hot sodium hydroxideextraction for a period of one hour at which point the pulp was againwashed.

A single stage hypochlorite bleach was then eifected, using thehypochlorite equivalent of 1% of the weight of the hulls. This step wascarried out at a temperature not in excess of 50 C. for a period of onehour, washing then being performed.

The charge at this juncture constituted high white pulp.

Purification was thereupon undertaken, comprising cold steeping for onehour using a 10% to 12.5% sodium hydroxide solution, followed bypressing, with the recovered solution being directed to storage forcooking liquor make-up, then by pulp shredding and washing.

The final brightening wash was thereupon made in hot water slightlyacidified, followed by a clear wash.

The pulp at this point was of high white brightness and had an alphacellulose content in excess of 96% and ash content of less than 0.1%.

As can be seen, even redwood and rice hulls, when processed by theimproved alkaline methods of the invention, provide without the returncycling steps ordinarily required, either a high white paper stock or,in the event purification is undertaken, a high yield of high qualityalpha cellulose. It is again emphasized that the improved process can beused with conventional cooking liquors of the soda or sulfate processes,with the departures described, and that the process of the invention iswholly compatible with the equipment presently used in the two alkalineprocesses.

What is claimed is:

1. An improved process of producing high white paper stock from woodcomprising the steps of adding to an alkaline process cooking liquorhaving a sodium hydroxide concentration of 64 to 80 grams per liter anda liquid to solid ratio of 5 to 1 to 6 to 1 a hypochlorous acid salt 1%to 2% by weight of the weight of the wood. of bringing the temperatureof the cooking liquor to C. to C. within one hour to one hour and aquarter, of maintaining said temperature for a period of one hour to onehour and a half, of blowing the digester, of. screening the pulp, ofsubjecting the pulp to a chlorination bleach, of washing the pulp, ofsubjecting the pulp to a hot sodium hydroxide extraction, of washing thepulp, of bleaching the pulp in a solution of sodium hypochloriteequivalent to 1% of the weight of the wood, and of washing the pulp.

2. The steps of claim 1 followed by purification comprising the steps ofsteeping the pulp in a cold solution of to 12.5% sodium hydroxide fromone half hour to one hour, of pressing said sodium hydroxide solutionfrom the pulp, of shredding the pulp, of washing the pulp and ofbrightening the pulp in a slightly acidified Warm wash, and of washingthe pulp to yield a purified high white pulp having an alpha cellulosecontent in excess of 96% by weight and with an ash content of less than0.1% by weight.

3. An improved alkaline process of producing valuable cellulosicproducts from rice hulls comprising the steps of digesting the hulls ina cooking liquor having a sodium hydroxide concentration of 80 grams perliter and a hypochlorite concentration of 2% by weight of the weight ofthe rice hulls, said digesting step including bringing the charge up toa temperature of 140 C. in a closed vessel in one hour and ofmaintaining said temperature for one hour and a half, of blowing,Washing and screening the charge, of subjecting the charge to a firstchlorination including introducing chlorine into the charge underagitation for 30 minutes at a rate of 0.25% of the rice hull weight perminute, of agitating the pulp for to minutes thereafter, of subjectingthe pulp to a hot sodium hydroxide extraction for one hour, of washingthe pulp, of bleaching the pulp using a solution of hypochloriteequivalent to 1% of the weight of the rice hulls, at a temperature notin excess of C. for one hour, and of washing the pulp.

4. The steps of claim 3 further characterized by a purificationoperation including cold steeping the pulp for one hour in a 10% to12.5% sodium hydroxide solution, of pressing, shredding and washing thepulp, and of brightening the pulp in slightly acid hot water bath.

References Cited in the file of this patent UNITED STATES PATENTS1,196,708 McCormack et al Aug. 29, 1916 1,235,258 Toles July 31, 19171,557,338 Runkel Oct. 13, 1925 1,570,389 Morgenier Jan. 19, 19261,773,320 Richter Aug. 19, 1930 1,858,523 Richter et al May 17, 19321,859,224 Swan May 17, 1932 2,249,174 Richter July 15, 1941 2,698,233Lewis et a1. Dec. 28, 1954 FOREIGN PATENTS 298,333 Great Britain Oct.11, 1928 471,695 Canada Feb. 20, 1951

1. AN IMPROVED PROCESS FOR PRODUCING HIGH WHITE PAPER STOCK FROM WOODCOMPRISING THE STEPS OF ADDING TO AN ALKALINE PROCESS COOKING LIQUORHAVING A SODIUM HYDROXIDE CONCENTRATION OF 64 TO 80 GRAMS PER LITER ANDA LIQUID TO SOLID RATIO OF 5 TO 1 TO 6 TO 1 A HYPOCHLOROUS ACID SALT 1%TO 2% BY WEIGHT OF THE WEIGHT OF THE WOOD OF BRINGING THE TEMPERATURE OFTHE COOKING LIQUOR TO 155*C. TO 165*C. WITHIN ONE HOUR TO ONE HOUR AND AQUARTER, OF MAINTAINING SAID TEMPERATURE FOR A PERIOD OF ONE HOUR TO ONEHOUR AND A HALF, OF BLOWING THE DIGESTER, OF SCREENING THE PULP, OFSUBJECTING THE PULP TO A CHLORINATION BLEACH, OF WASHING THE PULP, OFSUBJECTING THE PULP TO A HOT SODIUM HYDROXIDE EXTRACTION, OF WASHING THEPULP, OF BLEACHING THE PULP IN A SOLUTION OF SODIUM HYPOCHLORITEEQUIVALENT TO 1% OF THE WEIGHT OF THE WOOD, AND OF WASHING THE PULP.